Variable-frequency oscillator system



Aug. 31, 1948. M. R. GAvlN ETAL l 2,448,177

VARIABLE-FREQUENCY OSCILLATOR SYSTEM Filed Aug. 17, 1944 MALCOLM R. GAVIN A RNEY Patented Aug. 31, 1948 UNIT-ED STA-TES PATENT OFFICE VARIABLE-FREQUENCY osoILLA'roR Malcolm R. Gavin, Wembley, and Vincent A.

Heathcote, Fulham, London, England, assignors, by mesne assignments, to Hazeltne Research, Inc., Chicago, Ill., a corporation of 'Illinois Application August 17,7'1944, serial No. 549,840 In Great Britain August 27, 1943 7 Claims.

This invention relates generally 'to a variablefrequency oscillator sys-tem and particularly to an oscillator system which is useful in a frequencymodulation system. Specifically, the variablefrequencyl-oscillator system of the invention is effective 'to provide a relatively large frequency `change for a given control effect used to vary the frequency of the system.'

In one type of prior art variable-frequency oscillator system, an oscillator is provided having a frequency which is determined by a resonant circuit-used as the frequency-determining circuit for the oscillator, together with means, lfor Varying, in accordance with a modulation signal to be transmitted, the reactance of at least one element of the frequency-determining circuit, thereby to varyk the frequency at which the circuit is resonant and lin turn to change the operating frequency of the system. Arrangements of this type are effective t-o provide a relatively large Imodulation index.

change of frequency of the oscillatorto the change in the react-ance element of the frequency-determining circuit, is not as large asis desirable.

According1y,'lt is an object of the invention to provide an improved variable-frequency oscillator sys-tem.

It is la still further object of the invention to provide a variable-frequency oscillator system which has an improved sensitivity characteristic.

It is also an 'object of the invention to provide a variable-frequency oscillator system, including an oscillat-or the frequency of which is determined by a resonant circuit and `means for'varying the reactance of at least one element of the resonant circuit to vary the frequency of the oscillator, in which there is provided a relatively large ratio between the per cent. frequency shift which is provi-ded in the system for a g'iven per cent. change of the reactance of the frequency-determining circuit.

In accordance with the invention, a variablefrequency oscillator system comprises an oscilla- Vtor having a frequency-determining circuit portion including reactance elem-ents of both types connected in parallel to form a first circuit reso,- nant at a predetermined frequency, modulating means for varying the effective value of the reactance of at least one of the'above-mentioned types in the frequency-determining vcircuit to vary the frequency of the oscillator from the above-men- .tioned predetermined frequency, and means including a second parallel-resonant circuit having reactance elements of both types and a resist- However, for some purposes, the Asensitivity of the arrangement, or the ratio of the ance element and coupled to the first resonant circuit with substantially critical coupling so als to modify the reactance of the first resonant circuit greatlyto increase its sensitivity to the modu lation effects of the modulating means.

For a better understanding of the present intem lil includes a circuit comprising an inductorv L1 and a condenser C1 effectively connectedin parallel by means of coupling condensers l2, I3 and connected to the input electrodes of vacuum tube Il through-.a coupling condenser I 4 anda grid-leak resistor l5. In order to provide an oscillator having the circuit L1, C1 as the frequencydetermining circuit, the output electrodes of tube ll are incluctively coupled thereto by means of v-an inductor IB included in the anode circuit of tube'l l yandinductively coupled to inductor L1.

Thefvariable-frequency oscillator system li! also includes a means -for varying the effective value of Ithe reactance of at least one type lin thecircuit Li, C1 to vary the frequency of the oscillator from its predetermined ornormal frequency, as Vdetermined by the resonant frequency of circuit This last-named means includes a re-actance .tu-be 20 having its output electrodes coupled to circuit "L1, C1 lbymeans o-f a direct connection between the anode of ltube 20 and the common terminal of inductor L1 and condenser I3. Uni-directional operating potentials for tube 20 are provided by means of a source +B applied to the anode of tube 2wthrough a tap on inducvtor L1 and 'to the screen electrode of tube 2D through Va resistor 2|. A self-biasing circuit is provided for tube 20 by means of a resistor 22 in the `cathode circuit thereof, the resistor 22 being by-pas-sed for alternating currents by la condenser 23. In order tocause tube 2li to function as a reactance tube, a phase-shifting network, including a series-connected resistor 24 and a condenser 25, is connected between the anode of tube 20 and ground. The change in reactance effectively provided in circuit L1, `C1 by the tube 2li is 3 controlled by the application of a suitable control potential to the control grid of the tube.

The circuit so `far described is conventional and, in order to cause the variable-frequency oscillator system to function in accordance with the teachings `of the invention `to provide -a relatively high sensitivity, -a second resonant circuit including an inductor L2, a condenser C2, and a resistor R2 `connected in series in a loop circuit is inductively coupled to lcircuit L1, C1 by mean-s of an inductive mutual coupling M between 'indudtors L1 and L2.

The variable-frequency oscillator system of the draw-ing lis illustrated :as used in a frequencymodulation signal Itransmitter and thus also includes a modulation-signal source 28 coupled to the input electrodes of tube 20 and an amplifier 29 having an input circuit coupled to 'the output `electrodes of tube -I'l and an output circuit conneoted yto a balanced antenna system 3l, 32.

Considering now the operation of the system illustrated in the drawing, and neglecting for the moment the function of resonant circuit L2, C2, R2, the 'arrangement disclosed comprises a conventi-onal system in which tube H is included in an `oscillator having as a frequency-determining circuit the resonant circuit L1, C1, the signal output thereof being amplied in unit 29 and applied to antenna system 3|, 32 for radiation. Tube 20, dueto the phase-shift network 24, 25, when conductive causes a current to now through a portion of inductor L1 which is of such phase as eecltively to change the reactance in the loop circuit L1, I3, C1, I 2, thereby to change the frequency `of the oscillator system. The current which tube 20 provides in the circuit L1, C1 is controlled by means of the modulation-signal source 28, thereby to vary the frequency of the oscillator in accordance with the modulation signal to provide a frequency-modulated signal for transmission.

In accordance with the invention, the circuit L2, C2, R2 is inductively coupled to the circuit L1, C1 and is tuned t-o substantially the same resonant frequency as that of circuit L1, C1 'in the absence of a control signal applied to the input electrodes of tube 20. The circuit L2, C2, R2 has the effect of causing the system to have a greatly increased sensitivity; that is, a much greater change in frequency 4of the system is provided for a given change inthe input control signal for reactance tube 26. In order to show more fully why this is true, the operation of the circuit is hereinafter analyzed under two conditions, namely, (1) the condition Where M is zero, there- 'by eiectively eliminating the circuit L2, C2, R2

from consideration, and (2) `the `condition where M has a finite value, thereby to derive the conditions for optimum proportioning of the Acircuit in accordance with the invention.

The reactance of circuit L1, C1 is given by the expression:

where w=Angular frequency in radians 'per second. L1=Inductance value of inducto'r L1 in fhenries, L2=Inductance value of inductor L2 in henries. C1=Ca`pacitance value of condenser C1 in farads. lC'2=Capacitan'ce value of condenser C2 in farads. R2-Resistance value of v'resistor R2 in ohms.

M=Mutual inductance in henries between cire cuit C1, L1 and circuit C2, Lz R2 Where M :0, the circuit L2, C2, R2 has no eiect on the operation `of the circuit of Fig'. 1 and, thus, under the assumed condition, the arrangement of Fig. 1 functions as the prior are device mentioned above. In order to bring out the advantages of the circuit of the present invention, the circuit operation will rst be analyzed under the assumed condition so that the performance can be compared to that of the circuit of the invention. There-fore, in order to obtain the ratio of the change in frequency, under the assumed condition, to a change in the effective inductance of circuit C1, L1, consider the following:

At resonance 1 @afm 3) and 1 C1, L1, the expression j( is -still zero at resonance.

Therefore,

df( )=0 (6) And from Equation 5,

d wo: f( )/L14 (7) dLl @m0 But,

on w or L1 @.Fw (8) Also,

f( L f( w LVI- C0201 w @1110* 1 y L10 i- Tom-1; (9) -And since L10"w02010 fromEquation 3,

dit y dw @wu-2L10 (1o) Therefore, from Equations 7, 8 and l10 @l n dL1` 2L;o or

d600/.wo dL1/L10" `y2 (u) vEquation 11 therefore Vgives the ratio -of the change yin resonant frequency of circuit C1, L1 to the'change ininductance o'f circuit C1, L1 effected by the reactance tube 20 under the assumed c'ondition.

6 coupling. It has been found that-by approaching tl'iev critical coupling as nearly as=possible the change ,in resonant frequency ofthe system for a given change of reactance in circuit L1, C1 ca be increased vsome four or ve times. i f It is toy be understood that the arrangement of the invention is not limited to the particular oscillator circuitshown, but is applicable to any oscillatorA having a rifrequency-determining -lcircuit, the reactance vfof l'Which is controlled tdvary the frequency of the oscillator, it being lonly n ecc'ssary to provide insuch a ycase an additional quation 13 can be modied to obtain l @OMLZJFJ o --Expression 16 therefore gives the ratio of the change in the resonant frequency of circuit C1, L1 to the change in the inductance of the circuit C1, L1 effected by the rea'ctauce tube ZUunder the assumed condition. This'ratio is causedto have a large valuev When thev circuit of Fig.'1 is proportioned in accordance with the invention. Specifically,v Expression 16 is iniinite u nder the condition Where is approximately equal to and the condition in which they 4are equal will be referred to as the condition of critical Vand scope of the invention. f

tuned circuit corresponding to L2, C2, Rz coupled 'to the frequency-determining circuit of theA oscillator with substantially critical coupling. Wl'iile there has been described What is at Apresent considered to be the preferred embodiment of thisinvention,` it .will be obvious to those Vskilled in the art that various vchanges and modifications may be madetherein without departing from the invention, and' itI is', therefore,aimed in the'appended claims to cover all such changes and modifications as fall within ,the true `spirit l' What is'clamed is: j y l l 1. A variablefrequency oscillator system comprising, an oscillator lhaving a' frequency-determining: circuit portion including reactanceele- 'men-tsof both types connected in parallel to form a '.rst' f, circuit resonant at a predetermined frequency, modulating m'ans for varying the effective valueA of 'the reactance Iof atV least one of said types in said circuit to vary the frequency of said oscillator from lsaid., predetermined frequency and means including a second parallelresonant circuit havingreactanceelements of both types and a resistance element and coupled to said rst resonantcircuit with substantially critical coupling so as to modify the reactance lof said rst circuit-greatly to increase its sensitivity to -the modulationl effectsofl saidy modulating means.' 1

2. A variable-frequency oscillator system comprising, an oscillator having a frequency-determining circuit portion including reactance elements .of both types connected in parallel to form a first circuit resonant at a predetermined frequency, modulating means for varying the effective value of the reactance of at least one of said types in said circuit to vary the frequency of said oscillator from said predetermined frequency, and means including a second circuit coupled to said rst circuit with substantially critical coupling so as to modify the reactance of said rst circuit greatly to increase its sensitivity to the modulation effects of said modulating means said second circuit having reactance elements of both types and a resistance element and being parallel resonant at said predetermined frequency.

3. A variable-frequency oscillator system comprising, an oscillator having a frequency-determining circuit portion including reactance elements of lboth types connected in parallel to form a rst circuit resonant at a predetermined frequecy, modulating means for varying the effective value of the reactance of at least one of said types in said circuit tovary the frequency of said oscillator from said predetermined frequency, and means including a second parallelresonant `circuit having reactance elements of both types and a resistance element and inductively coupled to said first resonant circuit with substantially critical coupling so as to modify the reactance of said first circuit greatly to increase its sensitivity to the modulation effects of said modulating means.

4. A variable-frequency oscillator system comprising, an oscillator having a frequency-determining circuit portion including an inductor and a capacitor connected in parallel to form a rst .circuit resonant at a predetermined frequency,

modulating means for effectively varying the value of the inductance in said circuit portion to vary the frequency of said oscillator from said predetermined frequency, and means including a second parallel-resonant circuit having reactance elements of both types and a resistance element and coupled to said first resonant circuit with substantially critical coupling so as to modify the reactance of said iirst circuit greatly to increase its sensitivity to the modulation effects of said modulating means.

51. A variable-frequency oscillator system comprising, an oscillator having a frequency-determining circuit portion including an inductance L1O and a capacitor connected in parallel to form a first circuit resonant at a predetermined frequencywo, modulating means for varying the effective value of inductance in said circuit to vary the frequency of said oscillator from said predetermined frequency, and means including a second circuit having an inductance L2 and appreciable series resistance R2 and inductively coupled to said rst circuit by a mutual inductance M, said second circuit having parallel resonance at said predetermined frequency and being proportioned so that is substantially equal to 6. A variable-frequency oscillator system coniprising, an oscillator having a frequency-deter'- mining circuit portion including reactance elements `of both types coupled in parallel to form a first circuit resonant at a predetermined frequency, modulatingkmeans including a reactance tube connected with said frequency determining circuit for varying the effective value of the reactance of at least one of said types in said 'circuit to vary the frequency of said oscillator from said predetermined frequency, and means including a second parallel-resonant circuit having reactance elements of both types and a resistance element and coupled to said rst resonant circuit with substantially critical coupling so as to modify the reactance of said r-st circuit greatly to increase its sensitivity to the modulation effects of said modulating means.

7. A variable-frequency oscillator system comprising, a vacuum tube having input and output electrodes, a rst circuit having parallel reso'- nance at a predetermined frequency and including reactance elements of both types, means for connecting said input electrodes to said circuit and for inductively coupling said output electrodes to said circuit to form an oscillator in Which said circuit is the frequency-determining circuit, modulating means for varying the effective value of the reactance of at least one of said types in said circuit to vary the frequency of said oscillator from said predetermined frequency, and means including a second parallel'- resonant circuit having reactance elements of both types and a resistance element and coupled to said rst resonant circuit with substantially critical coupling so as to modify the reactance of said rst circuit greatly to increase its sensitivity to the modulation effects of said modulating means.

MALCOLM R. GAVIN. VINCENT A. HEATHCOTE.

REFERENCES CITED The following references are of recordin the file of this patent:

UNITED STATES PATENTS Bell et al. Sept. 24,1946 

